This invention relates to materials suitable for use in II-VI laser devices. Specifically, this invention is directed to a material for use in II-VI heterostructures and particularly heterostructures including zinc selenide.
In semiconductor laser technology, heterostructures are employed in order to reduce the threshold current for laser operation. These "heterostructures" are composed of materials of different band gaps (E.sub.g) which are arranged in a layer structure. It is very desirable to have the lattice constant of all the materials in the heterostructure identical, i.e., there should be no "lattice mismatch" at the interfaces. Lattice mismatch causes strain and misfit dislocations, and makes good epitaxial growth difficult. Certain III-V materials, used for lasers generating infrared and/or red light, such as gallium arsenide (GaAs) and gallium aluminum arsenide (GaAlAs) are inherently lattice matched so that lasers constructed from heterostructures of these two materials are relatively easy to manufacture.
Another desirable property of materials for heterostructures is chemical similarity, i.e., they should be made of elements from the same groups of the periodic table. This dramatically reduces any problems of cross-contamination between the layers, and improves the quality of epitaxial growth as well.
In the case of high band gap (E.sub.g&gt;2.6 ev) compounds, heterojunctions are usually made with elements from the right side of the periodic table. For example, ZnSe growth on ZnS.sub.0.2 Se.sub.0.8 is typical. Such materials are used to construct highly desirable blue generating devices. However, since the addition of sulfur (a smaller atom than selenium) reduces the lattice constant of ZnSe in the alloy crystal ZnS.sub.0.2 Se.sub.0.8 lattice mismatch occurs. In fact, there is no composition of ZnS.sub.x Se.sub.l-x or which lattice matching can occur. A larger atom, such as Te, could be added, but this reduces the band gap and thus offsets the advantage of adding it to some extent. The compound ZnS.sub.x Se.sub.y Te.sub.z, where x+y+z=1, would be useful for thin heterostructure layers (.congruent.1 um), but the thick material needed for substrates should have a higher band gap than is attainable with this compound. A similar result would be achieved by the substitution of Cd or Hg for Zn. Thus there is a need for the substitution of an element in ZnS.sub. x Se.sub.l-x, which will increase both the band gap and the lattice constant.